An elaborate compliment of sensors are available for sensing quantities such as altitude, air speed, and weight and balance of objects such as aircraft. One type of sensor for monitoring weight and balance of an aircraft is load sensor 10, FIG. 1. Load sensor 10 is basically a pair of inductors 12, 14 with a movable pole piece 13 between them. Such a load sensor 10 is described in "A Sensor System for Aircraft Weight and Balance," by Hal Nelson, Scientific Honeyweller, Fall 1987, pp. 91-95.
As shown in the schematic circuit of the load sensor 10, FIG. 2, when applying diametrically symmetrical sinewaves 21 to inputs 16, 18 of the load sensor 10, a null output at center tap output 19 is produced when the pole piece 13 is centered or balanced. As the pole piece 13 moves towards one of the inductors and away from the other inductor, the change in reluctance causes the output 19 of the load sensor 10 to increase in amplitude proportional to the amount of deflection; in the case of small deflections. The output 19 is in phase with the drive signal applied to the inductor to which the pole piece 13 is deflected towards. This action is analogous to an analytical balance.
In an additional idealized circuit of the load sensor 10 as shown in FIG. 3, each inductor 12, 14 is represented as an inductance and a resistance in series. The two inductors or coils are wired in series and an excitation voltage 23 at a predetermined frequency is applied across them at inputs 16,18. The output signal 24 is measured across inductor 12. In this arrangement, the output signal voltage 24 is proportional to the ratio of the inductor's 12 impedance to the total impedance of both inductors. When the target is in the center position, the coils have equal impedance and so the ratio is 0.5 as shown by the output signal voltage 24 being one half of the excitation voltage 23, FIG. 3. As the pole piece 13 approaches full scale deflection where it comes in contact with one of the inductors, nearly all the excitation voltage 23 is across one of the coils and the voltage ratio approaches 1.0. As shown in FIGS. 2 and 3, the load sensed by load sensor 10 is monitored by monitoring the output signals at outputs 19,24.
In a weight and balance system for an aircraft, in order to generate real time weight and center of gravity information regarding an airplane during loading, various load sensors 10 are attached to the landing gear and when such sensors are electrically energized they provide signals which are analogous to the amount of compression on the landing gear due to the load, which is in turn proportional to the weight thereon. As shown in FIG. 4, a plurality of load sensors N, like load sensor 10, are utilized in such a weight and balance system. When the outputs of these load sensors N are constantly examined by controller and control logic 47 via output multiplexer 46, aircraft weight and center of gravity can quickly be computed. Better loading configurations may then be realized resulting in a safer, better handling aircraft which requires less fuel to complete a given flight.
The weight and balance system 26 includes a driver scheme 28 for driving the load sensors N. As shown in FIG. 4, the output of a 10 Khz square-wave oscillator is filtered to provide a sinewave which drives a primary winding 34 of a small audio transformer 32. The audio transformer includes a secondary winding 36 having a center tap 38. This "phase-splitter" technique produces a pair of diametrically symmetrical sinewaves exactly 180.degree. out-of-phase with each other as shown by the sinewaves 21 of FIG. 2. The peak voltage of one of the sinewaves is compared to a reference voltage and the analog of the difference is fed back to oscillator 30 forming an automatic gain control loop 40. Each sinewave is amplified by amplifiers 41,42 and then applied to a particular load sensor of load sensors N via the drive multiplexer 43. Two feedback multiplexers 44,45 couple the drive outputs back to the respective amplifiers 41,42 in order to assure stability. The load sensor outputs are fed to a demodulator via an output multiplexer 46. A large amount of microprocessor control logic is required to operate the driver scheme 28, and also output multiplexer 46. With this driving scheme of weight and balance system 26, one load sensor is energized and monitored at a time.
A more simplified weight and balance system which measures moment of impact is shown by the sensing system 50 of FIG. 5. A small computer receives the output signals from two sensors 51,52. Because this system is only measuring moment of impact, only two load sensors 51,52 are required to be driven by driver scheme 71; the two sensors being for redundancy. All multiplexers of the weight and balance system 26 are eliminated. However, the always energized load sensors 51,52 require four drive amplifiers 59,60,61,62 instead of the two amplifiers required in the weight and balance system 26. The 10 Khz sinewave from oscillator 53 is applied to the primary winding 55 of the audio transformer 54. The amplifiers receive drive signals from the secondary winding 56 with center tap 57 of the audio transformer 54. Automatic gain control 58 is provided from the secondary winding 56 to the oscillator 53.
As shown in FIG. 4 and FIG. 5, the driver circuitry 28, 71 of both systems 26,50 respectively, include a large quantity of components. The weight and balance system 26 includes multiplexers and two amplifiers and the moment of impact system 50 includes four amplifiers. This inefficient driving scheme wastes energy, uses a large amount of board space and generates excessive heat. Because of these deficiencies, a need exists for a driver apparatus which eliminates excess components and provides a more efficient configuration for driving one or more sensors.